It is desirable to provide an indication of when the useful life of a perishable product has expired. Such perishable products include, but are not limited to, foods, food additives such as aspartame, biological materials, drugs, cosmetics, photographic supplies, and vaccines. One simple way of providing this indication is to mark each product with a suggested date by which the product should be used. However, there is a shortcoming in this method in that the actual useful life of perishable products is dependent on the temperature history to which the product is exposed because the rate of degradation of a perishable product usually increases with increasing temperature. In other words, a perishable product will generally have a shorter remaining useful life when exposed to a certain period at a relatively high temperature than when exposed to the same period at a relatively low temperature. More broadly, the rate of change of a particular property or characteristic of any material or product may increase with increasing temperature. Therefore, marking a product with a use by date must be based on assumptions about the anticipated cumulative thermal exposure of the particular product. However, the actual exposure cannot always be predicted or controlled. Therefore, there is a need to provide an indication of the useful life of a product taking into account the actual cumulative thermal exposure of the product by integrating the actual temperature exposure over time and providing a visual indication of exposure which equals or exceeds a predetermined allowable cumulative thermal exposure. Such an indicator should be capable of integrating temperature over time for the entire range of temperatures to which the product will be exposed and for the entire range of temperatures at which appreciable change in the product occurs.
Of particular concern is that the rate of degradation or other change at a given temperature is different from product to product, as is the variation in the rate of degradation with temperature. Some products show a greater increase in rate of change for a given temperature increase than other products. One useful way to quantify this is with reference to the Q.sub.10 of a reaction. The Q.sub.10 is an indication of how much faster a reaction (such as chemical change, microbial growth, or enzymatic spoilage of a perishable product) occurs in response to a 10.degree. C. increase in temperature: EQU Q.sub.10 =(Rate of change at T+10.degree. C.)/(Rate of change at T)
For example, most perishable foods stored under refrigeration have Q.sub.10 values based on spoilage by microbial growth which range from about 2 to 10. In other words, the rate of degradation will increase by a factor of from about 2 to 10, depending on the particular food, in response to an increase in temperature of 10.degree. C. Other perishable items such as drugs, biological materials, and vaccines will likewise show different Q.sub.10 values for each particular item.
The Arrhenius relationship is also a useful tool for quantifying the effect of temperature on many chemical and physical processes. The Arrhenius relationship is: EQU k=k.sub.0 exp(-Ea/RT)
where
k=the rate constant at temperature T;
k.sub.0 =the preexponential factor;
R=the ideal gas constant; and
Ea=the activation energy.
It is possible to perform experiments with particular perishable items to determine rates of degradation at various temperatures, and then apply the Arrhenius relationship to these experiments to calculate a measured activation energy (Ea) for each particular perishable item within a given temperature range. It has been observed that for many perishable items, such data will closely fit the Arrhenius equation, which assumes that Ea is independent of temperature. As with the Q.sub.10 value, the particular value of Ea will vary with the particular item to be monitored. For a further discussion on the analysis and quantification of the degradation of foods, reference is made to Theodore P. Labuza, Shelf-Life Dating of Foods 41-87 (Food & Nutrition Press, Inc. 1982).
Therefore, it is seen that there is a need to provide an indicator of cumulative thermal exposure in which the Q.sub.10 or Ea of the rate of providing a visual indication of cumulative thermal exposure can be approximately matched to the Q.sub.10 or Ea of the change of the object to be monitored. The indication of cumulative thermal exposure can thereby be approximately matched to the cumulative degradation of the object to be monitored.
It is also desirable to provide a time temperature indicator which has an unactivated state in which it may be stored at varying temperatures for long periods of time without changing. While it may be desirable to activate the indicator while it is being fabricated, the indicator also should be capable of selectively being switched to an activated state before, after, or at the time it is affixed to an object to be monitored, after a container is filed with contents to be monitored, after opening a container of contents to be monitored, or at any other desired time after the indicator is fabricated. Such an indicator, whether activated or unactivated, should not be deleteriously affected by environmental factors such as humidity and light.
Time and temperature indicators which visually indicate temperature exposure are known. For a discussion of several types of indicators, reference is made to Dee Lynn Johnson, Indicating Devices, in The Wiley Encyclopedia of Packaging Technology, 400-406 (John Wiley & Sons, 1986).
A time-temperature indicator which operates on diffusion properties and provides a visual indication by means of a chemical reaction is disclosed in U.S. Pat. No. 5,053,339, entitled "Color Changing Device for Monitoring Shelf-Life of a Perishable Product," issued to Patel (the '339 patent). The '339 patent discloses a color changing device for monitoring the time-temperature history of perishable products containing an activator tape and an indicating tape. The activator tape contains an activator composition matrix which includes an activating composition such as an organic acid. The indicating tape includes an indicating composition matrix which includes an indicating composition such as an acid-base dye indicator. One or both of the matrices is a pressure sensitive adhesive. The indicator produces a color change as the activating composition diffuses out of the activator matrix and into the indicator matrix and chemically reacts with the indicating composition in the indicating matrix. The color intensifies with time and temperature as more activator composition diffuses into the indicator matrix and reacts. Abstract, lines 1-17. Because the indicator disclosed in the '339 patent produces a color change based on pH, its operation is susceptible to changes in ambient moisture.
Another type of indicator is disclosed in U.S. Pat. No. 3,954,011, entitled "Selected Time Interval Indicating Device," issued to Manske (the '011 patent). The '011 patent discloses an indicator including a porous fluid carrying pad, a saturant material, a wick material for the saturant, and an indicator means whereby the progress of the saturant from the porous carrying pad along the wick material can be visibly indicated and used to measure the passage of time, the exposure to a given minimum temperature, or a time-temperature relationship. Abstract, lines 1-9. When the saturant is chosen so as to be in a liquid state at the intended storage temperature, the indicator indicates the passage of a time interval as the liquid progresses along the wick. Column 5, lines 12-21. The saturant may instead be selected so as to be solid at desired storage temperatures at which frozen foods are stored and to become liquid at temperatures at which the food is thawed. The saturant will remain solid while the indicator is at the desired storage temperature. The saturant will melt to a penetrating state and then progress along the wick while the indicator is above the predetermined temperature, thereby indicating the passage of time above the predetermined temperature. Column 5, lines 22-44. A plurality of saturant materials having varying freezing points, each having its own wicking path, can be used to indicate time of exposure to discrete predetermined temperature ranges. Column 5, line 45 through column 6, line 5. Such an indicator, however, is not able to record passage of time below the melt temperature of the saturant.
Another indicator is disclosed in U.S. Pat. No. 4,428,321, entitled "Thermally-Activated Time-Temperature Indicator," issued to Arens (the '321 patent). The '321 patent discloses a device which provides a visual indication that permissible time within a predetermined temperature range has been exceeded. Column 2, lines 912. The device includes an opaque microporous sheet with a colored stratum on its back and a transparent fusible coating bonded to its face. The fusible coating is a solid solution of an amorphous rubbery polymer dissolved in a crystallizable solvent such as wax. The solvent has a melting point below the lower end of the predetermined temperature range and the polymer has a glass transition temperature below the lower end of the predetermined temperature range. The solid solution does not appreciably penetrate the microporous coating. Column 2, lines 20-37. Below the melting temperature of the solvent, the composition is a non-penetrating solid. When the indicator is heated to a predetermined temperature, the solid wax solvent melts and dissolves the rubbery polymer, resulting in a liquid penetrating state which gradually penetrates the microporous layer. Column 2, lines 59-64; column 3, lines 23-28. Since the refractive index of the polymer and wax composition is essentially the same as that of the solid component of the microporous layer, the microporous layer gradually becomes transparent. Column 2, lines 64-67. The indicator disclosed in the '321 patent cannot indicate cumulative thermal exposure below the melting point of the wax solvent component of the coating. Furthermore, the '321 patent does not suggest applying the fusible coating to a backing for subsequent application to the porous coating, nor any other means for selectively maintaining the fusible coating out of contact with the microporous coating. Therefore, the indicator of the '321 patent must be stored below the melt temperature of the wax prior to use of the indicator.
There is no suggestion, however, to provide a time-temperature integrating device for providing a visually observable indication of cumulative thermal exposure, in which a viscoelastic material migrates into a diffusely light-reflective porous matrix at a rate which varies with temperature and which may be selectively switched from an unactivated state to an activated state over a wide range of temperatures.